1. Field of the Invention
This invention relates to a soot filter capable of removing soot from a gas, such as engine bleed air. More particularly, it relates to a filter capable of removing soot from bleed air from an aircraft engine, such as an auxiliary power unit.
2. Description of the Related Art
Modern aircraft are frequently equipped with an auxiliary engine referred to as an auxiliary power unit (APU). This is a small, on-board engine usually located in the aft section of an aircraft and used to power aircraft systems when the main engines of the aircraft are shut down. An auxiliary power unit can be used for various purposes, such as providing heat, electricity, air conditioning, and compressed air, but in contrast to a main aircraft engine, it is not capable of providing in-flight thrust. Compressed bleed air from the auxiliary power unit can also be provided to an air turbine starter system in order to start a main engine of the aircraft.
An auxiliary power unit is commonly a gas turbine, and like other types of gas turbines, it is susceptible to surging if the rate of air intake is improper for the engine operating conditions. Therefore, an auxiliary power unit is frequently equipped with a surge control valve which is fluidly connected to the auxiliary power unit and which is opened and closed based on operating conditions within the auxiliary power unit so as to prevent surging.
A surge control valve for an auxiliary power unit usually receives bleed air from the auxiliary power unit and performs control based on some characteristic (such as the pressure) of the bleed air. A surge control valve may include various pneumatic components, such as pressure regulators and pneumatically-operated valves, through which the bleed air from the auxiliary power unit passes. The bleed air frequently contains fine, tarry soot particles, i.e., unburned or incompletely burned hydrocarbons which are ingested by the auxiliary power unit from a variety of sources, such as from the diesel engines of ground service vehicles which service the aircraft when it is parked at a gate at an airport, from the exhaust of the engines of nearby aircraft, or from dust which is vacuumed up from the runway by the operation of the auxiliary power unit. The soot is extremely sticky, and if allowed to enter the surge control valve, the soot can cause the surge control valve to stick and malfunction. Other aircraft equipment which employs the bleed air from an auxiliary power unit may also contain components which are extremely sensitive to the presence of dirt, oil, and particularly hydrocarbon particles and which can be seriously damaged if they ingest soot particles with the bleed air. Accordingly, there is a need for a filter which can effectively remove soot from bleed air from an auxiliary power unit. The bleed air from the main engines of an aircraft may also contain soot, and since this bleed air may also be supplied to other aircraft equipment, there is a need for a filter which can effectively remove this soot as well.
Although filters exist which are capable of removing soot from a gas, existing filters are often impractical for use with aircraft engines because of various drawbacks, such as high costs, bulkiness, and insufficient dirt capacity, which necessitates their being frequently cleaned or replaced. Furthermore, because the bleed air for an auxiliary power unit typically has a very low flow rate, inertial separators are unable to effectively remove particles from the bleed air.